Process for producing colored regenerated cellulose film



1.963 J. M. FLETCHER ETAL 3,110,551

PROCESS FOR PRODUCING COLORED REGENERATED CELLULOSE FILM Filed April 10, 1962 AQUEOUS DISPERSION 0F SILOXANE POLYMER oxlblzluc BATH REDUCING BATH HUI EXTRUDER INVENTORS FLETCHER FOWLER JOHN MILFORD FRANK PIERCE ATTORNEY United States Patent 3,110,551 PROCESS FOR PRODUCING COLORED REGENERATED CELLULOSE FlLM John Milford Fletcher, Wilmington, DeL, and Frank Pierce Fowler, Kenmore, N.Y., assign'ors to E. L du Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed Apr. 10, 1962., Ser. No. 186,331 8 Claims. (Cl. 18--57) This invention relates to film, and more particularly to an improved process for producing colored, trans parent, regenerated cellulose films employing vat dyestuffs as coloring agents.

A process is disclosed in US. Patent 2,043,069 to Rusch, for preparing color-fast filaments and films by incorporating a pigment of vat dyestulf into a cellulosic solution, such as viscose, precipitating the cellulose from solution in the desired physical form, treating the freshly regenerated cellulose article with, first a reducing solution to convert the dyestulf to its leuco form, then an oxidizing solution to regenerate the dyestuff, whereby to develop the desired color uniformly throughout the article.

[Films prepared in this manner are well suited for use as tissue for wrapping a wide variety of delicate materials, such as foodstuffs, since the vat dye is present in the film as very finely divided solid particles which cannot be leached from the film even under severe conditions; hence, materials wrapped in colored film of this type are not contaminated by the dye.

Although colored films prepared by the above-mentioned process are quite satisfactory, the process for preparing them leaves much to be desired. On cast-ing viscose, pigmented with vat dyestuff as taught by Rusch, through a narrow metal die into a coagulation-regeneration bath to form regenerated cellulose film, particles of dyestuif and cellulosic material collect on the walls within the die opening, even though the particles are considerably smaller than the width of the die opening. The particles of dyestuff and cellulosic material adhere tenaciously to the inner walls of the casting die, or hopper, hence, accumulate and restrict the flow of viscose through the casting hopper. To compensate for the progressive build-up within the hopper in order to produce films of the desired uniform thickness, it is necessary to make frequent adjustment of the hopper opening. However, due to the non-uniformity of the build-up along the length .of the casting hopper, it finally becomes impossible to make film of uniform thickness by making further adjustments of the hopper opening; hence, it is necessary to halt the casting operation, open the casting hopper to scrape off and wash out the build-up of cellulosic mate rial and dyestuff. Such interruptions are quite costly, since there is a considerable loss of pigmented viscose, and the productivity of the casting machine is greatly reduced. In actual practice, it has been found that a cellophane casting machine can be operated satisfactorily for only about 2-4 hours after a hopper clean-up and that time lost during clean-ups reduced the productivity of a casting machine about 50-70%.

It is, therefore, the chief objective of this invention to provide an improved process for producing colored regenerated cellulose films of uniform thickness continuously for long periods of time without the necessity for frequent hopper adjustments to compensate for build-up of vat dyestuff and cell-ulosic material inside the casting hopper. The foregoing and related objectives will more clearly appear from the detailed description which follows.

These objectives are accomplished by the process of this invention which, briefly stated, comprises introducsolubilize (i.e., convert the dye to its leuco form) and more evenly distribute the dye throughout the film structure, and an oxidizing solution whereby to regenerate the leuco dye and develop the desired color.

The process of this invention is illustrated schematically in the accompanying drawing.

Because of their stability, ease of application, and availability in a wide variety of colors, the vat dyes derived from anthraquinone are the preferred dyestuff for use in the process of this invention. It should be pointed out, however, that the invention is not confined to the use of anthraquinone vat dyes, since the process may be employed to advantage in the preparation of regenerated cellulose film-s colored with indigoid, sulfur or phthalocyanine vat dyestuffs in the oxidized state. Care must be taken in the final oxidation step when a vat dye from either of the latter two classes is being used, since such vat dyes are subject to over oxidation which destroys the desired color. .To avoid this difficulty, an oxidizing agent considerably milder than sodium hypoohlorite should be used.

The silicone oil necessary for the success of this invention is'an organosilicone oxide (siloxane) polymer in which the structural unit is usually -R Si-.-O-, where R is a monovalent organic radical. Physical properties of the silicones depend on the size and type of radical (methyl, ethyl, phenyl, etc.), the RzSi ratio, and the molecular configuration of the polymer (linear, cyclic, degree of cross-linking). Of the wide variety of silicones which may be prepared, only those that are oils at temperatures below C. and are dispersible in an aqueous medium to yield stable emulsions are suitable for use in the process of this invention. The chemical composition of the silicone oil is not critical; however, polydirnethyl siloxanes are preferred, since they are commerically available in the form of emulsions which are quite suitable and convenient for use in the process of this invention.

Although it is preferred that the silicone oil be added to the vat dye dispersion as a pre-formed emulsion, if desired, the concentrated silicone oil may be used by adding it directly to the dye dispersion and emulsified by vigorous agitation in the presence of a suitable surface active agent. It is essential, however, that the vat dyestuff and silicone be thoroughly mixed together prior to the addition of the dye-stuff to the viscose. For best results, the dye dispersion should be constantly agitated to prevent settling out of the larger dye particles.

The exact manner in which the silicone oil functions in the process of this invention to prevent or inhibit build-up of dyestutf and cellulose within the hopper of a casting machine is not known; however, it is rather obvious, in view of the inert nature of silicone oils, that their effectiveness in this instance is a result of their physical rather than chemical properties. It is believed that the silicone oil forms a thin protective layer around the particles of vat dyestuff which prevent the dye particles from agglomerating and from being partially reduced by reducing agents present in the viscose thus precipitating a sticky, gelatinous layer of cellulose on the dye particles.

The optimum concentration of silicone oil to be incorporated into the vat dye dispersion is about 5% based on weight of the dye solids contained in dispersion. Although higher concentrations may he used, there is no added benefit to be derived from the increased amount of silicone oil, hence to do so is wasteful. Less than the optimum concentration of silicone oil may be used with corresponding less satisfactory results; however, it appears that there is no appreciable benefit to be derived if the silicone oil concentration is less than about 1.0%

For best results in carrying out the process of this invention, the vat dye dispersion should contain, in addition to the silicone oil, an oxidizing agent, such as sodium meta-nitrobenzene sulfonate, which acts as a sacrificial agent and is reduced in preference to the dyestufli by the viscose. The compounds found to be most suitable for this purpose are all aromatic compounds having at least one nitro group attached directly to the aromatic ring. Although sodium meta-nitrobenzene sul fonate proved to be most desirable because of its low cost and efiiciency, the following nitro-aromatic compounds were also found to be suitable for use in the invention: meta-nitroaniline, para-nitrobenzoic acid, B-S-dinitrobenzoic acid, 4-nitrotoluene-Z-sulfonic acid, dinitrost-ilbenedisulfonic acid, 1-nitronaphthalene-3,6,8-trisodium sulfonate, l-nitronaphthalene-o-sodium sulfonate, l-nitronaphthalene-7-sodium sulfonate.

In general, the preferred concentration range for the nitro-aromatic compound is about 25-75% based on the weight of the vat dyestulf; however the optimum concentration depends upon the particular nitro compound being used. For example, the optimum concentration of metanitrobenzene sodium sulfonate is about 45-50%; however, concentrations as low as about 5% have been used with some success. A concentration of 100% or greater may be used, but there appears to be no added benefits to be gained by doing so.

The effect of the silicone oil and the oxidizing agent I appear to be additive in the prevent of build-up of dyestulf and cellulose within a viscose casting hopper, since each agent acts in its own particular way to prevent reduction of the vat dyestulf and the resulting precipitation of gelatinous cellulose onto the dye particles. It should be pointed out, however, that the silicone oil is the only essential ingredient, since the effect of the oxidizing agent alone is not sufiiciently effective in inhibiting build-up and its action is merely supplementary to that of the silicone oil.

In the preferred embodiment of this invention, an aque ous dispersion is first prepared by combining the following ingredients in the proportions and sequence listed; percentages and parts are by weight:

Parts Anthraquinone vat dystuff (12% paste) 400 Soft water 1162 Sodium meta-nitrobenzene sulfonate (28.5% solution) 7O 35% emulsion of silicone oil (Dow Coming 3513;

aqueous emulsion ol polymethyl siloxane) 6 After the ingredients are thoroughly mixed, the resulting dispersion is injected by means of a metering pump into a stream of ripened viscose which is moving toward a cellophane casting machine hopper. The vat dyestutf and silicone oil are thoroughly dispersed throughout the viscose as the mixture passes through a pump which provides the force necessary to extrude the'pigmented viscose through the narrow opening of the hopper and into the coagulation-regeneration bath. The vat dye dispersion is introduced into the viscose stream at a rate predetermined to supply about 1.0% of solid dyestuff based upon the weight of cellulose contained inthe viscose. After the extruded viscose is coagulated into the form of a film and regeneration is completed, the moisture laden (gel) regenerated cellulose film is washed free of acid, desul: fured, then is passed through a sodium hydrosulfite solution to reduce solubilize and, more evenly distribute the vat dyestuff throughout the film. Subsequently, the film is washed free of excess reducing agent and is then passed through a solution of sodium hypochlorite to convert the dyestufi back to the stable, insoluble, oxidized form which has the desirable color and is tenaciously held within the regenerated cellulose film. After being washed, softened and dried in the conventional manner, the colored film is collected in roll form. a

The colored film prepared in this manner is nonbleeding and is, therefore, suitable for wrapping certain foodstuff and the like which do not require moistureproof wrapping. If, however, a moistureproof wrapping is desired or required, then this pigmented base film may be coated with one of the many moistureproofing coating compositions known to the art.

The following examples will serve to more particularly illustrate how the preferred procedure is carried out in actual practice and the benefits derived therefrom. The examples should not be construed as limitative however, since various changes and modifications may be made without departing from the spirit and scope of the invention. Parts and percentages are by weight unless otherwise indicated.

Example 1 fonate 70 Soft water 1162 35% emulsion of silicone oil (Dow Corning 35B) 6 1 Du Pont trademark for anthraquinone vat dyestuffs.

A continuous stream of this amber dye dispersion was pumped into a stream of ripened viscose as the viscose was moving toward a cellophane casting machine. The dye dispersion was supplied to the viscose at the rate necessary to provide about 1.0% solid dyestuff based on the weight of cellulose in the viscose. As the pigmented vispose tmoved onward toward the casting hopper, it passed through a pump, the mixing action of which thoroughly dispersed the added ingredients throughout the viscose.

After the viscose was extruded through the casting hopper into a sulfuric acid-sodium sulfate coagulationregeneration bath, the freshly formed gel film was washed, desulfured, then passed through a reducing bath containing 0.2% sodium hydrosulfite and 0.5% sodium hydroxide to convert the vat dye into the soluble leuco form, thus allowing it to be more thoroughly distributed throughout the gel film. l

The vat dyestuff was reprecipitated within the film as the insoluble oxidized form by passing the film through a 0.25% solution of sodium hypochlorite which also bleached the gel film to a lighter shade. The resulting amber colored film was then washed thoroughly to remove dyestulf loosely held to its surface, and the film was subsequently softened, dried and coated with a mois tureproof coating.

The casting operation was continued uninterrupted for five day-s to produce non-bleeding, amber colored film of good quality. During this production run there was only a slight deposit of the vat dye and cellulose within the casting hopper and only a few minor adjustments of the hopper opening were necessary to compensate for this build-up.

Example 2 Another dispersion of vat dye was prepared in a similar manner by combining the following materials:

. This dispersion of dye was injected continuously into a stream of viscose that was being processed into film as described in Example 1. There were no interruptions in the casting operation during a five day production run in which about 40,000 pounds of tango colored film of excellent quality was produced.

Example 3 A dispersion of vat dye was prepared in the manner described in Example 1 using the following ingredients:

Parts 11.0% paste of Ponsol Flavone GCN (Cl. Vat

Yellow 2) 220 14.0% paste of Ponsol Golden Orange RRT (C.I.

Vat Orange 2) 180 Soft Water 1162 35% emulsion of silicone oil (Dow Corning 35B) This dispersion of dye and silicone oil was introduce into a stream of viscose as described in Example 1. The casting machine Was operated continuously for two days before it was necessary to halt the operation to remove deposits from the casting hopper.

Example 4 A dispersion of red vat dye was prepared by combining the following materials:

Parts 18.0% paste of Car-banthrene 1 Red RR (C.I. Vat

Red 15 400 Sofit water 1162 25% solution of meta-nitrobenzene sodium sulfonate 80 35% emulsion of silicone oil (Dow Corning 1 Allied Chemical & Dye Corp. trademark for anthraquinone vat dyes.

This vat dye dispersion was used to produce red colored cellophane continuously for four days without any appreciable build-up of dyestuff within the casting hopper.

Example 5 Blue 6) 200 12.0% paste of Ponsol Jade Green F (C.I. Vat

Green 1) 7 7 Soft waiter 1162 25% solution of meta-nitrobenzene sodium sulfonate 80 35% emulsion of silicone oil (Dow Corning Example 6 A vat dye dispersion containing the following ingredients was used in the production of green colored cellophane for two days without any significant deposit of the dyestuff within the viscose casting hopper:

Parts 12.0% paste of Ponsol Jade Green P (C.I. Vat

Green 1) 112 11.0% paste of Ponsol Flavone GCN (Cl. Vat

Yellow 2) 228 Soft Water 1162 25% solution of meta-nitrobenzene sodiumv sulfonate 35% emulsion of silicone oil (Dow Corning 35B) For the sake of comparison, a control test was run to prepare colored film using the same conditions and vat dyestufif as in Example 2 except that the dye dispersion added to the viscose contained no silicone oil or metanitrobenzene sulfonate. Particles of dyestuif and cellulose collected rapidly in the casting hopper, hence it was necessary to make frequent adjustments in the hopper opening to produce film of the desired gauge. The casting machine was stopped for a thorough clean-up of the hopper after only two hours of operation, since it became impossible to compensate satisfactorily for the build-up of dyestuft in the hopper by making further adjustments of the hopper opening.

Another control test was run to produce colored film as in Example 2 except that only the silicone oil was omitted from the dye dispersion added to the viscose being cast. Satisfactory film was produced for 4 hours, then it was necessary to halt the operation to clean out the deposit of dyestuif from the casting hopper.

As pointed out hereinabove, the process of this invention makes possible the casting of vat dye pigmented viscose continuously for long periods of time with no appreciable build-up of dyestuff and cellulosic material within the casting hopper. As a result of this invention, the following advantages are realized:

(1) Casting machine clean-ups are less frequent.

(2) The productivity of the cellophane casting machine is greatly increased.

(3) Less frequent adjustments to the casting machine hopper opening are necessary.

(4) Colored film of more uniform gauge is produced.

(5) Less manpower is needed to operate the cellophane casting machine.

(6) Less pigmented viscose is wasted.

We claim:

1. A process for producing colored regenerated cellulose film which comprises introducing into viscose an aqueous dispersion of a sil'oxane polymer which is an oil at temperatures below C., and is dispersible in aqueous medium to yield a stable emulsion, and a vat dye, agitating the viscose to uniformly distribute the silicone oil and vat dye therein, casting the viscose into an aqueous coagulation and regeneration solution whereby to form a self-supporting [gel film of regenerated cellulose and thereafter treating said gel film in sequence with an aqueous solution of reducing agent effective to convert the dye to its soluble leuco form, and an aqueous solution of an oxidizing agent effective to regenerate the dye to its stable colored state.

2. The process of claim 1 wherein the dye is an anthraquinone vat dye.

3. The process of claim 1 wherein the siloxane polymer is polydimethyl siloxane.

4. The process of claim 1 wherein said reducing agent is sodium hydrosulfite.

5. The process of claim 1 wherein said oxidizing agent is sodium hypochlorite.

6. The process of claim 1 wherein the concentration of siloxane polymer in said'aq-ueous dispersion is within the range of from about 1% to about 5% based on the Weight of the dye solids in said dispersion.

7. A process for producing colored regenerated cellulose film which comprises introducing into viscose an aqueous dispersion of a siloxane polymer which is an oil at temperatures below 100 C., and is dispersible in aqueous medium to yield a stable emulsion, a vat dye, and a nitroaromatic compound having at least one nitro group attached directly to the aromatic ring, agitating the viscose to uniformly distribute the silicone oil, n-itro aromatic compound, and vat dye therein, casting the viscose into an aqueous coagulation and regeneration solution whereby to form a self-supportinggel film of regenerated cellulose and thereafter treating said gel film in sequence with an aqueous solution of reducing agent 8 elfectiive to convert the dye to its soluble le-uco form, and an aqueous solution of an oxidizing agent efie-e.ive to regenerate the dye to its stable colored state.

8. The process of claim 7 wherein the nitro-aromatic compound is sodium metamitrobenzene sulfonate.

. References Cited in the file of this patent UNITED STATES PATENTS 2,483,783 Polak Oct. 4, 1949 2,515,697 Cress-Well July 18, 1950 FOREIGN PATENTS 717,638 Great Britain Oct. 27, 1954 OTHER REFERENCES The Journal of the Society of Dyers Colourists, November 1955, vol. 71, pp. 641 to 644. 

1. A PROCESS FOR PRODUCCING COLORED REGENERATED CELLULOSE FILM WHICH COMPRISES INTRODUCING INTO VISCOSE AN 